spapr_vscsi: Set uninitialized variable

cppcheck reports this error:

hw/spapr_vscsi.c:274: error: Uninitialized variable: rc

If llen == 0, rc was indeed used without being initialized.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Alexander Graf <agraf@suse.de>